The present invention relates to a method for injection molding plastic parts by means of an injection molding machine, in particular a method for injection molding thermoplastic polymeric parts of any kind to any shape.
A conventional injection molding machine for medium and large scale manufacturing of plastic parts has a granular plastic feedstock material injection wherefrom plastic feedstock material is conveyed slowly towards an injection gate of an injection mold, e.g. using a conveyor screw or plunger. On its way to the injection gate the plastic feedstock material passes through a heating section so that it melts and can be injected under high pressure into the injection mold.
The molten plastic is injected in a shot, which is the volume needed to fill the molding cavity, compensate for shrinkage, and provide a cushion to transfer pressure from the conveyor system to the molding cavity. When enough material has gathered at the injection gate, the molten plastic is forced at high pressure and high velocity to run along sprue bushings/runners into the one or more cavities of the injection mold. These molding cavities of the injection mold are defined between an injector mold plate and a closely contacting opposite ejector mold plate that together delimit one or more molding cavities and confine the injected volume of melted plastic. The injection mold with its molding cavities is at a temperature below the solidification temperature of the plastic material injected. Pressure is maintained until the sprue at the injection gate solidifies so that no more material can enter the one or more cavities. Then the screw or ram of the conveyer system reciprocates the same distance as the screw or ram travelled forward when filling the one or more molding cavities, and acquires plastic material for the next cycle while the plastic material within the mold cools and solidifies so that it can be ejected in a dimensionally stable state. Such a conventional injection molding machine is e.g. known from International patent application no. WO 2012/055872.
Solidification can in some applications be assisted by means of cooling lines in the mold. A cooling medium, such as water or oil, circulates the cooling lines to achieve appropriate cooling. In such embodiments the mold is kept cold during shot injection so that solidification of the molten plastic feedstock material starts almost instantaneously at the beginning of filling the one or more molding cavities, the one or more molding cavities being identical or different. Once the required cooling temperature has been reached, the mold opens and ejector pins ejects/eject the solidified part(s) from the injection mold, and the process is repeated.
International patent application no. WO 2003/11550 discusses the most common ways to reduce the time required for the molding cycle. One stated way is to keep the temperature of a mold low to reduce the time required for cooling, however the disadvantages of this is that the surface quality of the molded part is worse than if slow cooling is allowed. Fast cooling also induces large residual stress in the molded part. So cold molding is not suited when molding e.g. thin parts along a long flow path. Such a resulting molded part is often uncomplete. A further problem is that too rapid cooling of a molded part within a cold mold can prevent crystallization of the resulting product thus deteriorating the quality of the final part. Injection can in some applications be assisted by means of heating the injection mold. This can be achieved by means of heating bars, heat films, cooling lines with a cooling medium (such as water or oil) circulating, etc. In such embodiments the mold is kept warm during the mold cycle so that the molten plastic flows easier when injected, but notably the mold is still cold enough to give ample solidification of the plastic during cooling when the cavity is full.
WO 2003/11550 speaks against a system where a cooling fluid is circulated alternately after circulating a heating fluid during a molding cycle, stating that such a machine and apparatus is quite complex, and also that the time required for a molding cycle becomes longer. Instead an integrated cooling shell is provided. The molding cycle involves injecting the feed at high pressure, as in other conventional methods and injection molding machines, and thus involves the resulting associated disadvantages, such as high pressure in the closed filled injection mold and need for high clamping force. Furthermore, the process requires considerable power consumption for both induction heating and for circulating cooling fluid, which contributes to making final molded parts very expensive. The use of specific layers and the induction heating also results in the molds wearing down fast.
Injection molds are generally made from tool steels, although stainless steel molds and aluminium molds are known to be suitable for certain applications. Aluminium molds have relatively short life time in number of mold cycles, but may though be preferred for low volume applications in conventional injection molding machines since mold fabrication costs are low and mold manufacturing time fast. It is e.g. known from German patent application no. DE 3017559 to use aluminium for a part of an injection mold, but not for the complete injection mold. For high volume production steel molds are better than aluminium molds because steel are not similarly prone to wear, damage, and deformation during the injection and clamping cycles, as aluminium. So mold materials are selected in view of duration, acceptable wear, the molds susceptibility to expansion when subjected to thermal fluctuations and changes, and to the plastic material intended to be used with it.
The Variotherm process proposes some remedies to the above disadvantages by using heating/cooling liquids in injection mold temperature control. The cavity wall of the mold is heated prior to injection of the melt to a temperature that exceeds the glass transition temperature of the melt. Then melt at is injected in the mold. It is explained that the already tempered mould surface is heated also by the hot plastic melt during the injection process, which indicates that the melt is at higher temperature than the injection mold, and thus of the heating liquid, so that onset of solidification can start early. After filling of the cavity the mold is cooled till the molded part has the necessary deforming temperature. As just emphasized the cooling of the hot plastic starts as soon as the melt enters the cavity, and then progresses with active cooling of the mold when the cavity has been filled completely, lasting until the plastic part has reached the required temperature for deforming. A reduction of up to 40% of the injection pressure is asserted offered by the Variotherm method. Also the clamping forces are indicated reduced. Despite that the Variotherm process was developed in 1970s the Variotherm concepts like oil heating/cooling and gas heating/cooling of injections molds are not widely applied. [A novel approach to realize the local precise Variotherm process in micro injection molding”, Lei Xie, Thalke Niesel, Monika Leester-Schädel, Gerhard Ziegmann, Stephanus Büttgenbach, Microsyst Technol., Springer-Verlag Berlin Heidelberg 17 Oct. 2012].
The company SINGLE Temperiertechnik GmbH, Ostring, Hochdorf, Germany has utilized the Variotherm process in The Alternating Temperature Technology (ATT). The cooling/heating channels of the injection molds are two separate closed, embedded in-mould circuits, SWTS circuits, that contain thermal fluid with different temperatures. Both SWTS circuits contain the same fluid. Water is recommended for temperatures of up to 200° C., while oil is suitable for very rare applications that operate with temperatures of up to 300° C. The system is equipped with an external valve station for switching the two circuits from bypass mode to mold temperature control mode. The circuits are made by Lasercusing, by building up layers of steel powder to form internal closed contour-aligned mold heating/cooling channels. The heating/cooling circuits of the molds that SINGLE uses for ATT are neither drilled or milled into the metal. Some of the disadvantages of ATT includes that these channels cannot be altered for e.g. width, accessed for cleaning if clogged, or inspected for defects, such as may occur due to erosion and pitting due to contact of mold metal with cooling/heating medium. Moreover the distance between the cavity and the circuit must be sufficient thick to avoid accidental breakage when pressurized heating/cooling medium passes through the lasercused circuits, when the mold plate are held forcibly together, and be able to resist injection pressure of the melt.
EP0335388 relates to a method of injection molding wherein the temperature of the injection mold is raised above the melting point of the plastic material through circulation of a heat carrier before injecting plastic material into the injection mold. The channels for the heat carrier are obtained by providing a gap between an insert and a cavity surface. The flow of the heat carrier is shot off through the injection mold upon injection of plastic material into the injection mold. After the cavity is filled with plastic material the injection mold is cooled to a temperature below the freezing point of the plastic material through suitable circulation of the heat carrier. The flow of the heat carrier through the injection mold is interrupted when injecting material into the cavity so that the temperature of the walls of those parts defining the cavity cannot be affected further, The passageways for the heat carrier are used for support during introduction of the material making the mold parts vulnerable for mutual displacement and leakage of heat carrier into gaps and cavities.
U.S. Pat. No. 5,423,670 discloses a similar device and method. During the preparation of the plastic material in the injection molding machine, the temperature of the cavity surface plates is raised to a level about the melt temperature of the material being molded. The tool cavity is warmed rapidly and uniformly before the material is forced into the cavity so that the injection pressures required to fill the cavity is reduced. Once the material completes filling the cavity a flow of coolant fluid cools the cavity insert plate by removal of the resident heat present in the cavity insert plate due to the preheating and the latent heat stored in the molten plastic. U.S. Pat. No. 5,423,670 provides no indication of suitable injection pressure.
WO00/74922 discloses a system and method that combine multiple opposing gates to reduce the meltflow pathlength and thereby reduce aspect ratio. The method includes non-isothermal steps of firstly, heating the mold surfaces with circulating heat transfer fluids supplied by a hot side supply system, to a temperature setpoint sufficiently high to retard solidification. Then secondly, injecting the melt through the opposing gates, then thirdly, rapidly cooling to solidification by circulating heat transfer fluids of much lower temperature, supplied from a cold side supply system. Each injection molding cycle thus starts with a heating phase, wherein the fast rise in mold surface temperature comes from a combination of high-thermal-conductivity metal (preferably, copper alloy) mold cavity materials, plus a very large thermal driving force being supplied by the hot side supply system fluid (preferably, steam). This fluid has a temperature well above the melt-solidifying temperatures (Tg or Tm) characteristic to the thermoplastic. The heating phase and injection is then followed by a fast cooling phase, wherein molding surface temperature decrease is thermally driven by cold side supply system fluid (preferably cold water) temperatures well below the melt-solidifying temperatures (Tg or Tm) characteristic to the thermoplastic. The greater these temperature differences are, the faster this “non-isothermal” molding cycle will be. To overcome the problems of poor mold surface replication for the molded thermoplastic article and to be able to maximize microreplication of the finest surface detail and contour the mold cavity part forming surfaces of WO00/74922 are heated at least above a characteristic solid-liquid phase-change temperature which is characteristic of the thermoplastic polymer. For amorphous thermoplastic polymers such as polycarbonate and acrylics, the preferred setpoint is the glass transition temperature (Tg). For crystalline thermoplastic polymers melting point (Tm) is proposed. The preferred setpoint temperature of the cavity surfaces is selected to be sufficiently high so that the thermoplastic part being molded is not formstable at any higher temperatures, so the hot side fluid need to be somewhat hotter than the setpoint to keep the cavity surfaces above the setpoint temperature. Then after the molding cavity has at least been completely filled by the molten thermoplastic and before the mold is opened at the parting line, mold surface temperature is dropped to below the Tg or Tm. WO00/74922 realises the problem of the plastic feedstock material being very stiff, so that it will require high injection pressure to fill the mold, thus also high clamping force to hold the mold parts in closed contact. The mold of WO00/74922 is designed for variable volume molding cavities and has in-mould heating/cooling channels to resist high claiming force and high injection pressure. The backside of the mold is not altered. Machining of said backside for making tempering channels would make it impossible to have the insulating air gaps needed to preserve the essential property of variable volume molding cavities, nor does WO00/74922 make proposals to suitable injection pressures.
Although some of the prior art briefly mentions reduction of injection pressure in relation to heating the injection mold prior to injecting the melt none of the prior art gives advice of which injection pressures are suitable. All the injection molds of the prior art suggested for such cooling system are complex structures, e.g. composed of many detailed components such as shells or parts kept distanced from each other by pillar, fins, blocks, distance pieces, etc., which creates a lot of turbulence and uncontrolled holding time of tempering fluid, or are embedded channels that are expensive to manufacture and cannot be inspected. All prior art mentions compromises for the higher temperature used when injecting, due to time and thermal energy required.
Depending on the mold material some prior art methods that rely on heat transfer and high turbulence or rapid flowing of fluid in metal channels, such as water, may cause erosion corrosion, flow-assisted corrosion, or even cavitation of the tempering channels, which inevitable will reduce lifetime of the injection mold. The metal material between the tempering channel and the mold cavity may deform and the tempering channels may even rupture or deform during the injection molding process, when subjected to both clamping force and injection pressure. Accurate design of the injection mold, control of the temperatures of the injection mold at various stages of the molding process, selecting the injection pressure and guiding the flow of the tempering medium appropriately is outmost important.